Always-on display
E-ink holds its image without power, so a clock on the screen is nearly free.
Battery
The always-on display uses its deep sleep state: the phone sleeps, wakes once a minute to paint the clock, and goes back to sleep.
Measured overnight cost: 3–4%.
The clock still updates every minute — the panel is repainted on each tick, and notification icons are refreshed in the same repaint rather than paying for a second one.
What shows on it
The clock, notification icons and the battery level. Waking the phone shows the lock screen with the notifications in full.
Turn it on or off with the Always on display tile in Quick Settings.
Notification icons are limited in number, with anything beyond the limit collapsing into a single overflow dot. That limit exists because each icon is drawn in the same once-a-minute repaint, and a row that never settles is a row that costs a refresh every time it changes.
Why it repaints at all
A bistable panel keeps whatever was last driven into it, including when the phone is off. That is why a black frame can persist through a reboot, and why the AOD needs an explicit repaint each minute rather than simply "staying on" — nothing stays on; the ink is just still there from last time.